// 快速排序
public class QuickSort {

    public static void swap(int[] arr, int i, int j) {
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
    
    public static void quickSort(int[] arr, int l, int r) {
        if (l < r) {
            swap(arr, l + (int)(Math.random() * (r - l +1)), r);
        }
        int[] p = partition(arr, l, r);
        quickSort(arr, l, p[0] - 1);
        quickSort(arr, p[1] + 1, r);
    }

    public static int[] partition(int[] arr, int l, int r) { 
        int less = l - 1;
        int more = r;
        while (l < more) {
            if (arr[l] < arr[r]) { // 当前数 < 划分值
                swap(arr, ++less, l++);
            } else if (arr[l] > arr[r]) {
                swap(arr, --more, l);                
            } else {
                l++;
            }
        }
        swap(arr, more, r);
        return new int[] {less + 1, more};
    }
}
